High potassium content in grape juice and wine are associated with low
quality red wine in warm wine producing countries. In an attempt to reduce
the potassium content of juice, must and wine, a field experiment was laid out
on the farms Meerlus and Kersfontein in the Paardeberg area near Wellington
in 1998 on granite derived soils to investigate the effect of canopy
management and fertiliser applications on berry K accumulation and wine
quality.
Four fertiliser applications, three canopy treatments and a MgSO4 foliar spray
were studied. The three fertiliser treatments being: none (control), CaSO4,
Ca(OH)2, and MgSO4 applications. The canopy treatments were: thin to two
shoots per bearer, tip, vertical shoot positioning (VSP) and the removal of
yellow leaves and lateral shoots (canopy 1), thin to three shoots per bearer,
top after véraison and VSP (canopy 2) and VSP with top after véraison
(canopy 3/control). Magnesium sulfate sprays were applied at véraison for
two seasons (1999/00 and 2000/01).
Seasonal effects produced the most significant differences in this experiment.
Canopy treatments did not affect juice K concentration at harvest. Canopy 1
and 2 produced significantly lower wine pH values at Kersfontein. Fertiliser
treatments had no effect on juice K concentration nor did it affect wine
quality. Magnesium sulphate foliar sprays did not affect juice K concentration at harvest but significantly lowered juice and wine pH, improved wine colour
density and total phenolic content.
It appears for this experiment that soil K content before véraison, shoot
growth at and after véraison and water stress after véraison were the main
factors determining juice K concentration at harvest.